Of course, it isn't as simple as that. The main problem is daylight savings time. The world doesn't switch at the same time - or does the Pacific coast switch to DST in the middle of the day? ;-) DST happens at different dates, different times, and with different differences in different timezones, and some zones don't switch at all.

On Unix, you can often get the time in a different zone by setting the TZ environment variable properly.

Abigail